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Have an Airmar shoot thru hull transducer connected to my Garmin 188C. Been having trouble right along with reading shallow depths but today the sounder portion of the machine failed to register anything. I checked the transducer and was surprised to have the top unscrew with very little effort. A quarter turn and it was off. The plastic housing that is cemented to the bottom of the hull was dry (empty). What kind of oil should I use to re-fill the base? Is it normall to have to refill the transducer with oil? Lastly, should the top of the transducer come off with so little effort? How can I check to make sure the transducer is working properly after the oil is replaced?

__________________ "I didnt claw my way to the top of the food chain to eat vegetables!"

if was me id be putting some water in the housing first to establish wether the housing is leaking seeing as to how it is now dry .. if it is leaking at the base and not from the top attachment ring .. cleaning the oil off to ensure a good bond will be more difficult than just drying the water ...the oil to use needs to have a viscosity that wont form air bubbles under boating conditions [giving u false readings] and be compatable with the ducer materials . glycerene is an example ..

Mineral oil or castor oil should be used to fill an in-hull base. These are thick liquids making them less prone to aeration. They do not evaporate as quickly as water and can also handle high power levels.

Mineral oil is what I have used in two installations. It won't evaporate so look for a leak.

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Lastly, should the top of the transducer come off with so little effort?

I believe it just turns and "clicks", it doesn't screw on.

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How can I check to make sure the transducer is working properly after the oil is replaced?

You have to put the boat in the water. Measure the depth with your anchor and rode or boat hook and compare it to the reading. If you get erratic readings underway but good readings at rest you may not have put enough mineral oil in the cup. The bottom of the transducer needs to remain submerged in the oil at all times.

Cleaned out the base for the transducer that is fixed to the inside of the hull. Filled it to the top with water this morning. Checked it this evening and it was down maybe 1/2 inch. If it is down more in the morning I guess I have a leak. If that is the case is there a way to seal it? Can I run a bead of epoxy or some other bonding agent around the inside of the base where it meets the hull?

__________________ "I didnt claw my way to the top of the food chain to eat vegetables!"

If the base was glued down with silicon the mineral oil will dissolve it and you will get a leak. If that is the case lift out the base and completely remove the silicon from the base and your bilge. Be sure the bilge is sanded smooth and cleaned thoroughly before re-gluing the transducer housing. Then re-glue the plastic base using 3M 4300 sealer adhesive. Place a large bead of the stuff on the base and push it into position. Wait 24 hours and then pour in mineral oil and reinstall the transducer.

Airmar changed the instructions.....they used to say...don't use silicone; now they say use silicone. I suppose it doesn't matter which you use. If you have a leak, you can use more of whatever was used to install the cone in the first place. I would use 4200 after cleaning the surfaces well....I'm not sure that 4200 will stick to mineral oil and if you had mineral oil (or any oil) in the housing then you need to make sure it's all gone before you try to seal it up. You might be better off trying to clean the outside and sealing from the outside rather than the inside.

If your P79 is turning loosely, I'd bet that someone forgot to put the rubber ring on it before they installed it in the housing the first time. That ring slips over the bottom of the unit and sits in a shallow groove to create a seal between the P79 and the housing I'm sure Airmar would send you a replacement if you asked nicely with the little package of rubber lubricant (also available from swimming supply stores). With that ring in place, you will have trouble even getting the P79 back on the housing...it's a real tight fit.

__________________ The views and/or opinions stated by the author in this post are only the views and opinions of the author

If it ain't broke, don't fix it. Nothing has changed on the P79 itself...only on the instructions. You will actually be just fine using either. I don't know why Airmar changed their instructions. My guess is that they found that 4200/5200 is somewhat brittle..and they found that silicone really isn't bothered by mineral oil these days, so they changed the instructions. That's no reason to go remount your P79 if it's already installed. Heck, mine is in with 5200 and they never recommended that...I just had it handy...and it works fine.

__________________ The views and/or opinions stated by the author in this post are only the views and opinions of the author

The 'brittle' theory sounds plausible. I figured the change was more to accomodate relocation - its obviously much easier to redo silicone vs 4200/5200. Its probably not uncommon that people miss that 'sweet spot' the first time around and have to move it.

As for which instruction is current, I think the answer lies in the statement 'Do not use an epoxy adhesive, polyurethane sealant, or the previously recommended Boatlife Life Seal or 3M 4200'.

5200 has been used in this type application sucessfully for years and I'm not suggesting it's improper. I just wanted to set the record straight on the dissolving silicone theory and Airmar's (apparent) current position.

Took jtburf's suggestion and filled the base with water mixed with red food coloring. The next morning I had some red water outside the base housing. Definately a slow leak. Called Airmar and they told me to use GE Silicon I or II. Ran a bead around the inside of the housing at the base. Cure time is 24 hours. Fingers are crossed. Here in SW Florida we felt the effects of all 4 hurricanes. Channel markers are gone and in some instances so are the channels. Really need the bottom machine up and running.

__________________ "I didnt claw my way to the top of the food chain to eat vegetables!"